1. Field of the Invention
The present invention relates to a fabrication method for a capacitor. More particularly, the present invention relates to a buried plate of a deep trench capacitor.
2. Description of Related Art
As semiconductor devices enter the deep sub-micron processing, the device dimension gradually reduces. For a dynamic random access memory (DRAM) device, the area for forming the capacitor also diminishes. On the other hand, as the application of software increases, the memory capacity required by a memory device gradually increases. When the demands on a smaller device dimension and a larger memory capacity become higher, it is obvious that the fabrication method for a capacitor of a DRAM device needs to the modified.
The structure of a DRAM capacitor is mainly divided into two types. One type is the stack capacitor, while the other is the deep trench capacitor. For a deep trench capacitor, increasing the capacitance of the capacitor within a limited area can be achieved by increasing the contact area of the electrode. Therefore, a bottle-shaped deep trench structure is typically used in a deep trench capacitor. Since the bottle-shaped deep trench can increase the area of the buried plate, the capacity of the capacitor also increases.
FIGS. 1A to 1E are schematic, cross-sectional view diagrams illustrating the process flow for fabricating a buried plate of a deep trench capacitor.
Referring to FIG. 1A, a substrate 100 is provided, wherein a patterned mask layer 101 is formed on the substrate 100. The mask layer 101 comprises an opening which exposes a surface of the substrate 100. Using the mask layer 101 as an etching mask, an etching is conducted to pattern the substrate 100 to form a deep trench 102. An oxide layer 104 is further formed on the surface of the deep trench 102, except the top part of the trench 102. A nitridation process is subsequently conducted to form a silicon nitride layer 106 on the surface of the exposed substrate 100 in the deep trench 102.
Referring to FIG. 1B, the oxide layer 104 is removed. Thereafter, wet etching is conducted to form a bottle-shaped deep trench 102a, wherein the part of the sidewall of the deep trench that is covered with a silicon nitride layer 106 is precluded from being wet etched.
Continuing to FIG. 1C, the silicon nitride layer 106 is then removed. A conformal doped layer 108 is formed on the surface of the substrate 100 and on the surface of the deep trench 102a. A deep trench 102a is then filled with a photoresist layer 110, covering the doped polysilicon layer 108, wherein the photoresist layer 110 does not completely fill the deep trench 102a. 
Referring to FIG. 1D, the conformal doped layer 108, not covered by the photoresist layer is removed, leaving the doped layer 108a at the bottom of the deep trench 102a. The photoresist layer 110 is subsequently removed. A thermal process is further conducted to drive in the dopants in the doped layer 108a into the substrate 100 to form a doped region 112, wherein the doped region 112 serves as the buried plate of the deep trench capacitor. Thereafter, the doped layer 108a in the deep trench 102a is removed as shown in FIG. 1E to complete the fabrication of a buried plate of a deep trench capacitor.
In accordance to the above fabrication method, to complete the fabrication of a deep trench with a bottle shape structure requires multiple processing steps. Further, the bottle shape structure and the doped region (buried plate) are formed in different process steps. Therefore, the conventional fabrication process is very time-consuming. Moreover, the uniformity of the thickness of the photoresist layer, which is used to control the dimension of the buried plate, is difficult to control during the fabrication process. Consequently, the capacitance of the capacitor in the memory devices is not consistent. Further, forming the silicon nitride layer on the sidewall surface of the substrate is accomplished through a nitridation reaction. However, the desired thickness of the silicon nitride layer is difficult to control through a nitridation reaction. As a result, the silicon nitride layer formed according to the prior art is not effective in preventing the erosion of the etchant.